Sound motion pictures have historically employed a variety of types of soundtracks, including variable density optical tracks and magnetically-recorded tracks.
The present invention is concerned with a third type, namely variable area optical tracks. Such soundtracks are currently recorded in accordance with accepted industry standards.
At the present time, the motion picture industry uses analogue soundtrack with such a variable area density strip, carried alongside the film image. To read such a track, present practice requires an incandescent exciter lamp as a source, a small projection lens system and a slit to image the exciter lamp on to the analogue area of the film soundtrack. The received radiation passing through the film soundtrack is picked up by a photosensitive detector and amplified, with the amplified signal being connected to appropriate loud speakers, etc. Thus, the detected signal becomes the soundtrack.
A major disadvantage and difficulty with this system is that the exciter lamps have to be changed frequently on each film projector. This is a skilled and labor intensive operation, because the lamp current and the physical position of the lamp filament need to be adjusted each time a lamp is changed, in order to maintain sound quality. After changing a lamp a test soundtrack, known in the industry as Buzz Track, is run. As there is no zero base line on variable area soundtrack, the use of an automatic gain control system (AGC) is precluded. This would have been an obvious way to correct for lamp and detector aging.
This known prior art system suffers from a number of disadvantages. It requires a lot of maintenance and scrupulous cleanliness, whilst providing less than optimal signal - to - noise ratio. Further, in general, such a system requires a high degree of shielding from extraneous ambient light, lest such light degrade the output signal.
Proposals have been made for alternative soundtrack reading systems. U.S. Pat. No. 4,085,296 (Keegan) discloses a modulated sound reading system. Here, a light emitting diode is the excitation source. It is modulated at a carrier frequency several times higher than the highest frequency component of the soundtrack. The light beam transmitted through the soundtrack is detected by a photo-sensing device in an analogue format. The output signal of the photo-sensing device is demodulated and fed into a signal processing circuit to draw out the audio signal.
This proposal suffers from a number of disadvantages. First, it requires an elongate slit adjacent the LEDs, and hence has all the problems of aligning various optical components to ensure that they are properly aligned. Further, in this arrangement, the detector is a phototransistor or solar cell, which would not be suitable for reading a digital soundtrack. It simply senses the overall intensity of the light passing through the soundtrack.
U.S. Pat. No. 4,124,784 (Johnson et al) discloses a motion picture sound system. Here, a light source, e.g. a conventional filament lamp, and an aspheric lens are provided for illuminating the soundtrack. This arrangement is intended to produce a narrow band or strip of light that illuminates a corresponding narrow band of the soundtrack. On the other side of the film strip, there is a focusing lens and a detector having an array, preferably a linear array, of photosensors. The problem with this arrangement is that it requires careful alignment and adjustment of the various components. The narrow strip of light focused onto the soundtrack must be well focused; if it is blurred, then this will result in degradation of the reproduced soundtrack. For good reproduction of the soundtrack, it is essential to focus the hot spot of the lamp filament, as present in the narrow band of light, onto the detector array. This is difficult to achieve consistently. Maladjustment of the focus of the slit image on the soundtrack will result in loss of high frequency components from the material.
A further potential problem, which becomes important as the width of the slit is decreased in an attempt to provide high frequency response, is the production of Fresnel fringes in the image of the slit on the soundtrack. When the detector "sees" these fringes, a further loss of audio high frequency content will occur. By using an incandescent exciter lamp, the effect of the Fresnel fringes is reduced, because the classical light and dark bands associated with these fringes developed by monochromatic light are replaced by alternating colored bands of light. As the typical detector used responds nearly equally to different colors, the effect is merely to broaden the image of the slit, again resulting in sound degradation.
Another problem associated with the current state of the art detectors in film theater operation is that the light output of an incandescent lamp changes, with changes in the electrical current flowing through its filament. With time, the filament material is evaporated and deposited on the inside of the glass envelope. Light transmission losses between the lamp and the light detector varies with the dirt contamination on optical surfaces. As these surfaces are close to the film threading path, such contamination is inevitable. As mentioned, automatic gain control is not possible, and for current lamp technology, the sound reproduction system is provided with an adjustable variable current power supply. The only way to compensate for these normal variations is to adjust manually the exciter lamp current. This is known the art as an open loop control system and requires operator intervention.
This alignment of the optical path and changing light intensity of the light detector are sources of major service items in theater operation.
In U.S. Pat. No. 4,698,787 there is disclosed an optical type information reproducing device. This particularly concerned with reproducing information from a laser disc, and as such is concerned with a somewhat different area of sound recording. It includes a so-called laser pick up device comprising a plurality of signal reproducing PIN diodes.